With the increased packing density of heat generating elements now obtainable in an electronic assembly, thermal design considerations have taken on greater significance. In order to provide sufficient heat dissipation, passive cooling may have to be augmented by a forced convection system using a blower fan or other source of a cooling gas stream. Typically, the cooling gas is air, but design considerations may sometimes dictate the use of another gas. In the following description, reference to "air" should thus be construed broadly to include alternative gasses.
When a fan or blower is used, air borne contaminants are brought into contact with the components of the electronic assembly at a rapid rate, sometimes decreasing performance or resulting in an increased likelihood of malfunctions. Indeed, the dirt problem is so significant that direct contact, forced air systems are not generally permitted within electronic assemblies destined for use in military applications. See, for example, MIL-STD 454.
To avoid the dirt problem and still provide the desired degree of forced air or convection cooling, resort may be had to an indirect cooling technique which uses a "cold plate". In such an arrangement, a heat conductive, usually metal cold plate is disposed adjacent to the electronic assembly so as to define a narrow, usually sealed, region or gap between one surface of the plate and one surface (usually bearing the heat generating elements) of the assembly. Heat generated in the assembly is carried to the cold plate, generally by radiation, conduction and/or free convection. A fan or blower is arranged to generate an air stream adjacent to the other (exterior) surface of the plate, so as to effect heat exchange between the plate and the forced air stream. Since the air stream does not directly enter the gap or contact the assembly, the dirt buildup problem is thus eliminated.
While the use of a cold plate has satisfactorily enabled cooling of electronic assemblies without dirt contamination, the amount of dissipation provided by this technique is sometimes insufficient. To increase dissipation, it is often necessary to increase the air movement capacity of the fan, increase the heat transfer areas, (i.e., enclosure size, cold plate size), or to use heat sinks, all with added costs. In addition, enhanced dissipation can be achieved by increasing the efficiency of heat transfer from the assembly to the cold plate, typically by introduction of a very small amount of air flow in the gap. By limiting the air velocity in the gap to an amount which is less than or equal to that associated with free convection between vertical parallel plates, significant dirt contamination is still avoided.
The latter alternative is sometimes difficult to implement, since devices which produce low volume air flow (such as miniature fans or piezo-electric resonators) are often expensive. Introduction of a small opening in the head end of the gap region (between the assembly and the cold plate) so as to produce a small positive air pressure differential in that region may work satisfactorily, but this arrangement requires that the same air source be used for both cooling of the exterior surface of the cold plate and for the introduction of a low velocity flow in the gap region, which may be a disadvantage in some circumstances.
In view of the foregoing, it is the broad object of the present invention to provide efficient cooling of electronic assemblies without contamination by dirt or dust. In particular, it is desired to achieve the advantages attained by use of a cold plate, while significantly increasing the amount of heat which may be dissipated. It is also desired to induce a low velocity gas flow in the gap between an electronic assembly and a cold plate using the same fan or blower used to cool the exterior surface of the cold plate, while permitting a different gas source to enter the gap.